Hyperthyroidism

Valerie L Ward, MD
Nayer Nikpoor, MD

September 3, 1996

Presentation

Imaging Findings

Images obtained from technetium-99m-pertechnetate (TcO₄) thyroid scintigraphy show abnormally increased homogeneous radiotracer uptake throughout the thyroid, which is normal in size. The intensity of thyroid gland uptake exceeds the uptake in both salivary glands (arrows), background activity is markedly decreased, and the pyramidal lobe is clearly visible. All of these findings indicate a hyperfunctioning gland. There is no focal photopenic or focal hot area to suggest a nodule.

Differential Diagnosis

Diagnosis

Discussion

The radiotracer uptake by this patient's hyperplastic and hyperfunctioning gland is uniform and intensely increased in the right lobe, left lobe, and isthmus. The pyramidal lobe, which normally has little or no tracer uptake, is also hyperplastic and demonstrates increased uptake. Therefore, the clinical manifestations and abnormal thyroid function tests correlate with the scintigraphic imaging findings.

Technetium-99m-pertechnetate (TcO₄) is an efficient radiotracer for thyroid scanning. It is trapped and concentrated, but not organified, by the thyroid. The advantages of TcO₄ are several: it is readily available from a molybdenum-99/Tc-99m generator, it is taken up by the thyroid in 20 minutes, and it has a short physical half-life of 6 hours. This radiotracer provides a lower radiation dose per unit administered than any of the radioiodines (I-131 and I-123) that are used for thyroid imaging. A disadvantage of TcO₄ is its lower target-to-background ratio than the radioiodines. For most patients, the advantages of TcO₄ outweigh its disadvantages.

Preparation of the patient prior to thyroid scanning is important. All medications that may interfere with the thyroid's uptake of the radiotracer are discontinued. Female patients must be asked whether they are pregnant or breast feeding. Thyroid scanning is contraindicated in pregnancy, especially in the first 12 weeks, because it causes suppression of the fetal thyroid tissue. The radiotracer is secreted in human breast milk, so breast feeding must be discontinued for 24 hours after the scan. Following patient preparation, TcO₄ is injected intravenously using a dose of 10 mCi. The patient is placed supine with the neck extended. A gamma camera is used with a 3 to 6 mm aperture pinhole collimator (for magnification) and a 20% window centered at 140 KeV. Twenty minutes after injection, images are obtained in the anterior, right anterior oblique, left anterior oblique, and bird's eye projections for 200K to 250K counts. The bird's eye view gives an overall view of the thyroid (from the chin to the manubrium) in relation to its surrounding structures. This view also allows comparison of tracer uptake in the salivary glands to the thyroid gland. In this patient, uptake in the thyroid exceeded uptake in the salivary glands, which is consistent with hyperthyroidism. Thyroid scintigraphy with TcO₄ provides practical information about the functional status of the thyroid gland.

References

2. Mettler FA, Guiberteau MJ. Essentials of nuclear medicine imaging. 3rd ed. Philadelphia: Saunders, 1991: 75-94.

3. Thrall JH, Ziessman HA. Nuclear medicine: the requisites. St Louis: Mosby-Year Book, 1995: 321-343.

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